The signaling mechanism of phyA involves direct interaction with ATG8 to regulate HY5 autophagic degradation under nutrient starvation.

Phytochrome A (phyA), the only far-red light (FRL) photoreceptor, initiates photomorphogenesis under FRL. Autophagy, an evolutionarily conserved degradation pathway, facilitates plant adaptation to nutrient stress. Recent studies revealed that elongated hypocotyl 5 (HY5) undergoes autophagic degradation during carbon and nitrogen starvation, a process antagonized by cryptochrome 1 (CRY1) through its binding to autophagy-related 8 (ATG8). The present study investigated how phyA engages with autophagy to mediate FRL signaling under nutrient starvation in Arabidopsis, a process whose mechanisms remain unclear. We combined protein-protein interaction, genetic, phenotypic, autophagic degradation, transcriptomic, and cellular localization assays to investigate this process. We demonstrate that autophagy-deficient mutants atg5, atg7, and atg8n exhibit enhanced photomorphogenesis under FRL. We further show that phyA physically interacts with ATG8 to suppress HY5 degradation via the autophagy pathway during combined FRL and nutrient starvation. Moreover, phyA restrains the nuclear export of ATG8e and inhibits autophagosome formation. Collectively, our results identify a phyA-ATG8-HY5 regulatory module that orchestrates photomorphogenesis under nutrient deficiency. These findings, together with earlier reports on CRY1, illustrate how distinct photoreceptors employ divergent strategies to converge on autophagy and fine-tune HY5 stability, thereby optimizing plant growth in fluctuating light and nutrient environments.